Humidity sensing element



Jan. 26, 1965 B. L. BRUCKEN ETAL HUMIDITY SENSING ELEMENT 2 Sheets-Sheet 1 Filed Nov. 27, 1959 INVENTORS Byron Z. Bracken V/cfor A Mil/iami/is Wm THE/R ATTORNH/ Jan. 26, 1965 B. L. BRUCKEN ETAL 3,167,734

HUMIDITY ssusmc ELEMENT Filed Nov. 27, 1959 2 Sheets-Sheet 2 1/ H W z //8 I A64 4 I I i (m i 1 #Q m E 7' IN V EN TORS TH R ATTO/F/Vfy United States Patent (3 3,167,734 HUMIDITY SENSING ELEMENT Byron L. Bnuclren and Victor A. Wiilianiitis, Dayton, Ohio, assignors to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed Nov. 27, 1959, Ser. No. 855,589 8 Claims. (Cl. 338-495) This invention relates to a domestic appliance and more particularly to an improved clothes dryness control circuit and dryness sensing element.

The clothes drying art has long sought an accurate clothes dryness control which will automatically terminate a drying cycle when the clothes are at the desired degree of dryness. This problem is made more difficult by the fact that a dryer is confronted with loads of varying size containing many different fabrics. In the copending application Serial No. 823,763, filed June 29, 1959, now Patent No. 3,088,221 issued May 7, 1963, there is set forth a dryer control in which the dryness of any given clothes load may be sensed during a mechanically controlled test period of reduced air flow and reduced heat input. This invention incorporates a similar test period in an electronically controlled test period arrangement.

Much work has also been done in the past on humidity sensing elements which sense the relative humidity of air passing thereover as the means to control a circuit. However, the prior art devices were such that the electrical current generated or passed by the sensing element required amplification before it could be used in a control circuit. Equipment necessary for such amplication adds materially to the cost of the control and thus the earlier humidity sensing elements are not acceptable for the domestic appliance field.

The prior art sensing elements fall mainly into two categories(l) an element in which the hygroscopically sensitive material is coated on or developed on the surface of the element and (2) an arrangement whereby the hygroscopic material is mixed mechanically with the element. in the first arrangement wherein the surface only is sensitive, the element is incapable of conducting current in the magnitude required to operate an electrical control directly. Such surface sensitive elements produce signals in the microarnps range and are comprised merely of a plastic body in which the plastic molecules remain Wholly separate from the ionizing group coated on the surface thereof. Here, the ionized current flow is on the surface only. On the other hand, the mechanically mixed ionizing mixture of category (2) is unstable. This latter arrange ment results in the hygroscopic material migrating to the surface of the element where the moisture attracting ma terial is exposed to the leaching action of the gaseous mediums passing thereover. During the life of the element such leaching changes the resistance characteristics of the element.

This invention distinguishes over either of the prior art elements design in that the hygroscopic material and its carrying base are chemically compounded into a mass ionizing body. This structure, in which the hygroscopic material and its carrying base are chemically compounded is capable of conducting currents sufiicient to operate relays in the milliamp category, i.e. directly without need for further amplification. In other words, the mass ionizing arrangement of the chemical compound will handle 500 to 609 times more current than will the equiva- 3,167,734 Patented Jan. 26, 1965 lent configurations constructed along the lines of surface ionizing combinations.

Accordingly, it is an object of this invention to provide an improved humidity sensing element.

A further object of this invention is the provision of a humidity sensing element comprised of an electrically nonconducting support portion covered with a thin film of plastic material which is chemically treated to be somewhat hygroscopic itself or impregnated with hygroscopic salts, the electrical resistance of which element varies with relative humidity.

it is also an object of this invention to provide embedded in the last thin film a bi-filar arrangement of electrical conductors in spaced relationship which will reflect a potential across the conductors indicative of the elative humidity of the air surrounding the element.

Another object of this invention is the provision of a humidity sensing element comprised of a compound of hygroscopic material for supporting electrical conductors embedded therein.

A further object of this invention is the provision of a method for forming a humidity sensing element in which moisture ionizing film is molded about a conically shaped mandrel; to become its own self supporting structure.

it is also an object of this invention to provide a control circuit for a clothes dryer which incorporates the humidity sensing element of this invention.

it is a broad object of this invention to provide, in a control circuit for a dryer, a humidity sensing element which will operate an electric control directly.

A more particular object of this invention is the provision of a clothes drying cycle in which a humidity sensing element senses the relative humidity of exhaust air and selectively actuates an electronic timer, an air flow reduction means, and a heating arrangement to reduce heat input to the dryer during an electronically timed test period.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are clear ly shown.

in the drawings:

FIGURE 1 is a schematic side sectional View of a clothes dryer suitable for use with this invention;

FIGURE 2 is a side elevational view partly in section of one embodiment of the humidity sensing element of this invention;

FIGURE 3 in a side elevational view with the humidity sensing element in section and exploded to show the method of forming the humidity sensing element of FIG- URE 2;

FIGURE 4 is a side elevational view partly in section of another embodiment of the humidity sensing element of this invention;

FIGURE 5 is a schematic control circuit for a clothes dryer which incorporates the humidity sensing element of this invention; and

FIGURE 6 is a modified arrangement for installing the humidity sensing element in the circuit of FIGURE 5.

In accordance with this invention and with reference to FIGURE 1, a clothes dryer It) is shown comprised of a casing 12 havim a front opening 14 closed by a door 16. Within the casing 12, a tumbling drum 18 is adapted to be rotated by any conventional means to tumble clothes or fabric in the path of air circulated by a blower 29. Both the blower 20 and the tumbling drum 1% may be rota-ted by a motor 104 installed within the casing 12 of the dryer. A rear wall 22 of the drum 18 is perforated to place the interior of the drum in communication with a heating element having a low wattage section 1% of approximately 1600 wa ts and :a relatively high wattage section 1% or" 2800 watts. At the front of the drum 13 an access opening 24 is provided in axial alignment with the dryer door 16. The opening 24 communicates with a front duct 26 which connects through an exhaust duct 27 to an exhaust opening 28 in the rear wall of the dryer casing 12. Inlet openings 30 are formed in the rear wall of the casing adjacent the heating elements 1% and 1%. Thus, the energization of the blower 26 will set up an air circulation through the tumbling drum as follows: air will enter the dryer casing 12 through the inlet openings and will be heated by the heating elements 166 and 108, which may be energized together or separately to vary the heat input to the entering air. The heated air will then flow through the perforated rear wall 2?. of the tumbling drum 18 and exit from the tumbling drum by way of the access opening 24 at the front thereof. Air will follow the front duct 26 toward the exhaust opening 28 as induced by the blower 20. While passing through the tumbling drum l8, moisture will be entrained in the air flow and transmitted through the duct work 25 and 27 to the exhaust opening 28.

Disposed within the exhaust duct 27 is a humidity sensing element 154. The humidity sensor 154 is one component of a humidity sensing assembly 1G2 which in addition to the element 154 includes a potentiometer 156 andia relay 153, as shown in the circuitry of FiGURE 5. The potentiometer 156 affords the operator of the dryer a means to select the desired end point dryness of the fabrics being dried. The relay 153 receives the signal sensed by the element 154 and is operated directly thereby in view of the novel construction of the element to be described more fully hereinafter. It should be understood that the humidity sensing element 154 may be placed in positions other than the vertical position shown in FIGURE 1. It is important only that the element 154 be disposed in a manner within the moisture laden exhaust air stream to receive an indication of the relative humidity of the air being transmitted thereover.

One of the problems in correctly sensing a moisture condition of air is the response of the element to a rapidly flowing current of air. To increase the cont ol sensitivity of the system it is desirable .to reduce the amount of air flow past the humidity sensing element during certain periods which will hereinafter be referred to as test periods. During a test period the heat input to the tumbling drum 18 is reduced as well and this arrangement of reduced air, reduced heat will concentrate the remaining moisture in the reduced volume of air flowing past the humidity sensor. If the moisture or relative humidity of the air climbs above a predetermined figure, say 40% R.H., before the test period expires, the dryer will again be conditioned for full air flow and full heat and the drying cycle will continue. To accomplish these ends, a damper 170 is installed within the exhaust duct 27 and adapted to be controlled by a solenoid 164 in the dryer control circuit (FIGURE The damper 17%) includes a small weep hole or bleed opening 4% which allows a small regulated amount of air flow to pass over the humidity sensing e1ementl54 even when the damper 17% is close The operation of the dryer 10, briefly stated, is as follows. Damp clothes are placed through the access door 16 into the drum 18. The dryer is turned on to energize one or more of the heating elements 166 and his to heat up the air passing through the tumbling drum. At the same time, the motor 1494 is energized to rotate the tumbling drum 18 and the blower 2% During the first part of the drying cycle when the relative humidity is below approximately 25% R.H., the damper 17% will be closed and the heater 1% energized to provide rapid heat up within the tumbling drum. This also prevents the humidity sensing element 154 from giving a false indication of the dryness prior to the time that suflicient moisture is entrained in the passing air to give the sensing element 154 a true picture of the clothes dryness condition. As soon as the exhaust air indicates a moisture condition or high relative humidity, above approximately 40% Rl-l, the damper 170 will be opened and the full input including both heaters 1% and 1% will be energized to give maximum input to the dryer. As the moisture is vaporized from the tumbling fabrics, the clothes tend to become dry. After a duration of time, depending on the fabric and thsize of the load, the relative humidity of the circulating air will fall and the humidity sensing element 154 will transmit this indication of dryness through the humidity sensing assembly 1%2, which will program a test period as follows. The damper 170 will close to restrict air flow and the heater 1% only will be energized. If the moisture in the air passing the element 154 fails to increase above its predetermined setting during the test period, the cycle will be terminated automatically and the clothes Will be found in the dryness condition selected by the operator. A more complete description of the clothes dryer operation will be given hereinafter in connection with FIGURE 5.

An important provision of this invention is embodied in the design of a humidity sensing element which uses a mass ionization technque to reflect a moisture condition and produce a signal in the milliamp range to directly control a relay or other electrical control component. structurally, the element 15 is seen in FEGURE 4- and is comprised of a support member 543 of plastic or other suitable electrically non-conducting material. The support 5% is cylindrical in cross section and is provided with an integral peripheral flange 52 on one side of which is an electrical terminal 54 and on the other side of which is an electrical terminal 56. Overlying the support portion 59 is a moisture sensitive film 58 in which are embedded spaced conductors so and 62. The conductors as and 62 are spirally wrapped about the cylindrical film 5S and embedded therein. One end st of the conductor 62 is attached to the terminal connector 54 while the end 66 of conductor 6% is attached to the terminal bolt or connector 56. Thus, there is provided a pair of spaced conductors embedded in a moisture sensitive film to form a bi-filar humidity sensor. Depending on the amount of moisture in the air surrounding the element 154, a certain amount of ionization occurs within the film 53 and a potential is developed between the conductors so and 62. Such signal is transmitted through a potentiometer to a control relay, as will be described in connection with the control circuit of FlGURE 5 hereinafter.

Note that in FIGURE 4 a carrier or cylindrical support is provided on which the moisture sensitive film 53 is disposed. However, in this arrangement only the exterior surface of the moisture sensitive film is exposed to air passing thereover. In FZGURES 2 and 3 there is shown a modified version of the humidity sensing element 154. In this arrangement a hollow cone-like humidity sensing element is comprised of a moisture sensitive film 76 which is molded into the desired configuration. As in the embodiment of FIGURE 4, a pair of electrical conductors '72, '74 are wrapped in a spiral configuration about the element 68 and embedded within the moisture sensitive film 7s. Here again, the terminal ends of these spaced conductors are connected to terminal connectors '76 and '73 which may be used, in turn, to electrically connect the element into a control circuit. A base 73 carries the term nals 76 and 7 3 and also supports the element as. Note that in the element 6 the moisture sensitive film 7 it may be exposed on both its exterior and interior sides to air flow passing thereover. in this arrangement it is desirable to place the element d3 so that its axial or longitudinal axis is in the direction of air ilow such that air will pass through the core of the element.

One method of forming the element 68 is shown in FIGURE 3 and includes a conical mandrel 80 over which the moisture sensitive film '7 may be molded. The conical shape of the film 70 is provided solely to allow for sufiicient draft to permit withdrawal of the element 68 from its molding mandrel 89.

The mass ionization techniques of this invention are accomplished by chemically bonding or retaining an hygroscopic material in a supporting film capable of attracting moisture. The ionizable material is homogeneously dispersed throughout the film to place the ionizable material in a continuous phase, i.e. the ionizable particles are so closely contiguous in the dried out sensing element that they become continuous when moisture is absorbed by the element. The film or sensing portion of the element may be comprised of acrylic plastic, polyamide plastic or rubber (natural or synthetic) latex. Phenolic plastic varnish is also believed useful as the carrier substance for the sensing film.

The acrylic plastic which may be used for film 58 in the element 154 or film 779 in the element 68 is formed from a short simple caustic soda treatment of a mixed polyacrylonitrile film to render the element moisture sensitive. Specifically, a one hour hot treatment with two percent sodium hydroxide on Lecton coated magnet wire gives a very substantial change in resistance as relative humidity varies. To extend the usefulness of this reaction, a spiral of parallel Nichrome wires 60, 62 on a nylon tube 58 was varnished with the Lccton polymer. This chemical compound was found to be stable against leaching (that characteristic which causes the hydroscopic material to be withdrawn from its carrying film) and corrosion and yet yields a resistance from 500 ohms to 27,500 ohms on a humidity change of 85 down to 25 relative humidity.

The Lecton polymer is apparently a tri-polymer of acrylonitrile, methacrylic acid and butyl methacrylate with a preponderance of acrylonitrile. A possible representation is The sodium hydroxide treatment can place sodium ions at any one of three molecular positions, i.e. the nitrile group (CEN) via hydrolysis; the acid group via neutralization; and the ester group via saponification. Gnly a relatively small portion of the possible replacements are made, since complete replacement would yield a water soluble polymer and this is not desired. Along this same line, a different approach would be by co-polymerizing sodium acrylate with acrylonitrile and other acrylic monomers if wider latitudes of resistance change are required. The polyacrylamides would work as well or better than the polyacrylics.

The following are representative examples of acrylic films suitable for use with the humidity sensing elements of this invention.

EXAMPLE I-ACTIVATED LECTON FILM To the cylinder 5'!) of the element 154 apply Du Pont Lecton varnish No. RK6304. Bake the element and the coating 58 applied thereon for one hour at 320 F. After the baking operation, the film is activated for one-half hour in two percent NaOH at 150 F. Subsequently, the film is dried for one hour at 250 F.

EXAMPLE II-POLYACRYLATE FILMS Film A Dilute a solution of Acrysol Al (Rohm & Haas) to fifty percent of polyacrylic acid. Coat this solution on the cylinder 50 to form a film and then dry at 250 F. for one-half hour.

Film B (zinc polyacrylate) Prepare a solution of zinc acetate and NH OH. Coat the cylinder 5t of the element 154 with this solution and dip the coated element in a ten percent solution of Acrysol ASE (ammonium polyacrylate). Allow the element to dry at 250 F. for one-half hour.

Film C (calcium polyacrylate) Polymerize calcium acrylate onto the surface of the element cylinder 50. Allow the polymerized film 58 to dry in an atmosphere of 320 F. for one hour.

One characteristic disadvantage of humidity sensors is embodied in a tendency of the sensors to change resistance as the element ages, thereby impairing the life expectancy of the element. The plastic coating or carrier film 58 or 70 tends to separate slightly (due apparently to aging and shirinkage) from the wire electrodes or conductors 60, 62 and the conductivity at high humidity decreases. Leaching of the electrolyte, such as sodium hydroxide, and a change of moisture sorption as the film ages may have much to do with this disadvantage.

Other plastic coatings were developed to overcome the above lite disadvantage and a polyamide plastic film proved to be a desirable solution. These alcohol soluble nylons were selected in an attempt to get more stable films that would not separate from the electrodes or conductors such as 69 and 62. Chemically. the nylons are polyamides and treatment with electrolytes such as NaOH or even salt such as NaCl can introduce the metal ions at some of the links in the polymer chain. This may happen either by splitting olf some of the chains to give some O Na structure or perhaps by the potential chelating effect of the amide structure to tie up some of the metal ions. In either case, it was established that while the nylon alone had a weak conductive effect, this was very measurably enhanced by treatment with a sodium hydroxide solution. Addition of stearic acid, sodium stearate, sulphuric acid, sodium oxalate, nickelous nitrate, copper acetate and even calcium sulphate were tried in the alcohol soluble type of nylon. All were potent in giving a reactive element with a longer life expectancy than that found in connection with the acrylic plastic film. A variation of the polyam-ide films to get more conductive ions therein includes concentrated monomers of acrylic acid, calcium acrylate and sodium acrylate which were polymerized in place over the electrodes. Reaction to changes in humidity are evident in this construction as well.

The following are representative examples of polyamide films suitable for use as a coating on the humidity sensing element of this invention.

EXAMPLE IH- ACTIVATED NYLON Apply to the element cylinder 50 Du Pont Zytel No. 61 nylon (ten percent solution in a solution of parts methanol to 15 parts water). Bake the coated element for one-half hour at 320 F. Follow the baking with a two-minute fusing at 400? F. wherein the element is heated until a melting, glazing or smoothing appearance is observed for the film. Activate the film for one-half hour in a two percent NaOH at 150 F. Allow the element to dry and again fuse the film for two minutes at 400 F.

EXAMPLE IV-NYLON ADDITIVES To a twenty percent solution (85 par-ts to parts methanol water) of Du Pont Zytel No. 61 nylon, add between three and five percent of any one of the following: stearic acid, sodium stearate, H 80 HCl or fifty percent CaSO Allow the film to dry and fuse at 400 F. for two minutes.

Another film, taught more fully in concurrently filed copending application Serial No. 855,756, now Patent No. 3,073,161 issued January 15, 1963, which has been found satisfactory for use with the elements 63 and 154 concerns itself with rubber latex rather than plastic films for the conductive membrane between electrodes. The constancy with extended life tests was superior to the plastic films described hercinbefore. One suitable rub ber latex was a milkweed latex coating, but other natural rubber and synthetic rubber latexes have been found to work even better. Further compounding with electrolytes, such as sodium chloride, improves the conductivity of the natural rubber latex and its life appears substantially better than either the acrylic or the polyamide plastic films. enhanced if the element is not exposed or baked out at temperatures above 140 F.

The activity of the rubber latex films comes from a pure physical retention of natural and/or added electroiytes rather than a chemical compounding as is the case in connection with the acrylic plastic and the polyamide plastic films. A typical composition of natural rubber latex is:

Percent Water 67.0 Rubber hydrocarbon 29.5 Protein 0.9 Acetone extract (Sterolesters, fatty acids and phyposterol) 1.0 Quebrachitol 1.1 Sterol glucosides 0.07 Ash 0.45

In this arrangement or composition, the ash contains the natural ionizing electrolytes such as KCl, K PO Na PO etc.

EXAMPLE V-NATURAL RUBBER LATEX FILM To a natural rubber latex comprised of 62.5% solids, 0.25% Santobrite, and 0.15% ammonia, add water to produce a 50 parts natural rubber latex to water mixture. Apply this mixture on the cylinder 50 of the element 154 and allow the film to dry three hours at 140 F.

EXAMFLE VlNATURAL RUBBER LATEX AND ADDITIVES Prepare a latex-water mixture. Saturate the water with any one of the following materials or groupings: NaCl, (NaCl and quaternary ammonium salt), Nal, (KNO and NaCl), Cal-IP0 NaBr or Ca (NO Prepare a 50-50 mixture of natural rubber latex to saturate water. Apply this latex-water mixture to form a film on the cylinder 50 of the sensing element 154. Allow the film to dry for three hours at 140 F.

Although all the foregoing examples were recited as being coated on the support cylinder 50 of the element 154, it should be recognized that substantially the same films could be used to form the humidity sensing element 68 iliustrated in FIGURES 2 and 3.

The foregoing acrylic, polyamide and latex films may The life expectancy for the latex film is be compounded with any suitable electrolyte. Any acid, base or salt which is ionizable will produce the desired condition, however, sodium or potassium hydroxide are believed most useful for commercial purposes. With regard to the polymerized films, it is within the purview of this invention to react the electrolytes before or after polymerization.

Even though the first two films described are chemically bonded to the current carrying ion whereas the latex film physically retains the electrolyte, all three films use a mass ionization technique wherein the presence of moisture is reflected throughout the entire film rather than at the surface only thereof. The utilization of this mass ionization technique gives rise to a current carrying capacity for the humidity sensitive elements in the order of milliamps. Such current may be used to operate an alternating current relay or other relay device directly in a control circuit for a clothes dryer as will be described more fully next following.

The sensing element 63 or 154 is adapted to be used in an electronic timed control circuit shown in FIGURE 5. Power source for operating a dryer with the above control circuit may be a three-wire 230-volt source having leads L1, L2 and a neutral N. The circuit includes an electronic timer arrangement 1%, a humidity sensing assembly 1&2, the motor 104 for rotating the tumbling drum and the fan 20, and a heater having a 1600-watt portion 106 and a 2800-watt portion 108. Safety devices are provided in the form of a door switch 110 whichshuts down the dryer whenever the door is open and a centrifugal switch 112 which is closed whenever the motor 104 is in operation. Temperature control is provided through a temperature responsive thermostat 114 and a safety high limit thermostat 116.

In operation a control cycle is started by depressing a start switch 18 which causes power to flow through the closed door switch 110 to the motor 104, thereby to start the tumbling operation and close the motor switch 112. A holding circuit is established by solenoid coil 109 and its switch blade 111. At the same time a solenoid 164 is energized to close the damper 170 and connect the heater 1% across 115 volts for the reduced air flow, rapid heatup start of the drying cycle.

The electronic timer 100 is comprised of a gas diode 124, a diode rectifier 12%, a capacitor 128 and a timing resistor 130 to regulate the charging rate of the capacitor 128. Another resistor 132 limits the amount of discharge through the diode 124 to protect the bleed contacts 134, 136. A cycle termination solenoid 140 is included in the electronic timer and adapted to motivate a drying cycle termination switch blade 142 for opening contacts 144 to terminate the drying cycle. The electronic timer acts, when energized, to store a charge in the capacitor 128 until it reaches the firing point for the gas diode 124. At this point current will flow through the solenoid 140, thereby opening the switch 142. Note that when the contacts 134, 136 are closed by a charge discharging switch 145 of a relay 1:18 in the sensing element assembly 102, the timing circuit will be shorted to ground and any charge collected will be dispelled as follows: from line 140, the resistor 132, the relay switch blade 146, line 150, line 152 and solenoid coil 164 back to neutral. The time that it takes for the capacitor 128 to store a charge sufiicient to reach the firing point of the gas diode 124 is known as the test period. The duration of this test period may, of course, be varied.

The sensing element arrangement 102 is comprised of a sensing element 154, a potentiometer 156 which is variable by the user of the dryer to predetermine the degree of dryness desired, and an alternating current relay 158 which is actuatable by the mini-amperage passed by the sensing element 154.

In operation the first air to pass the sensing element 154 in a drying cycle will be relatively dry and the sensing element 154 will, thus, have little moisture coming in contact therewith. The resistance of the element will be high and the relay 158 will remain deenergized. The air flow reduction switch portion 160 and the charge discharging switch portion 146 of the relay will be positioned as shown in FIGURE and current will flow from L1, line 120, line 162 through the relay switch 160 to a damper and heater solenoid 164 and from there to the other side of the line N. Thus, the relay 164 will close its switch blade 166 and open switch blade 168 to place the heater segment 106 across 115 volts to secure 1600- watt operation. At the same time, the damper 170 will be placed in its phantom line closed position to restrict air flow through the dryer. This restricted air flow will, in effect, concentrate the moisture within the dryer air system and the sensing element will be exposed to high humidity. As the humidity sensed by the element increases, additional current will flow to the relay 158. During this interval power also flows to the electronic timer 190 by way of L1, line 120, line 162, line 150 and a charge is stored in the capacitor 128. If the charge collected by the capacitor equals the firing point on the gas diode 124, the relay 140 will be energized and the switch blade 142 opened to deenergize the start solenoid 109 which, in turn, opens switch blade 111 to terminate the drying cycle.

However, in normal operation while the clothes are still damp the sensing element 154 passes current due to increased humidity to energize the relay 158 and to open the switch blade 160 before the electronic timer is able to fire. Each time the relay switch 160 opens, the solenoid 164 will be deenergized to open the damper 170 and place the heaters 166 and 108 in series across 230 volts for maximum heat operation. Also, each time that the element 154 senses a humidity above 40%, the energization of the solenoid 158 will close its switch 146 to bleed off the charge from the capacitor 128. This shunt circuit is by way of line 149, resistor 132, relay switch blade 146, line 150 and line 152, solenoid coil 164 back to N, the neutral side of the line.

During a normally sized clothes load approximately two cycles of the electronic timer 100 are required to complete the drying cycle. That is, the sensing element 154 will indicate a dry condition to the solenoid 158 which will be deenergized to initiate the electronic timed test period. During this test period, the damper 170 will be closed and the heaters connected for low Wattage operation during the period that the electronic timer 100 is building a charge on the capacitor 128. If the capacitor 128 acquires a charge sutficient to discharge the diode 124 before the sensing element 154 picks up enough humidity to energize the solenoid 158, the solenoid 140 will be energized to open the switch 142 and terminate the drying cycle.

An alternative humidity sensing arrangement 180 is shown in FIGURE 6 and includes the same sensing element 154 set forth hereinabove and the potentiometer 156. The bridge circuit shown generally at 182 rectifies the alternating current to direct current for operating the direct current solenoid 184 which may then control the switches 146 and 160 as described in connection with FIG- URE 5. The circuitry of FIGURE 5 will apply completely as describedthe only change being in the adaptation of the humidity sensing arrangement 180 for the arrangement 102 shown in FIGURE 5.

The dryer termination control described in connection with FIGURES 5 and 6 is adapted to utilize the element 154 or 68 to conduct a sufiicient electrical signal to operate a relay directly. Note that no amplification is needed in producing a signal which is suitable for actuating an electronic circuit for controlling the dryer components. The control system is adapted to sample the relative humidity of exhaust air between approximately 35% RH. and 40% RH, and to continue the drying cycle if the relative humidity climbs above 40% RH. in a given test period, or to terminate the drying cycle if the relative humidity does not increase. The electronic timer is added to the circuit merely to effect a timed test period of approximately four or five minutes during which time the element 154 senses the humidity condition or" the exhaust air as an indication of clothes dryness. If relative humidity fails to rise above approximately 40% in the predetermined test period, the electronic timer is arranged to deenergize the dryer and terminate the drying cycle.

While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. A humidity sensitive element comprising a cylindrical support member having an integral annular flanged base portion, a first electrical terminal on said base portion, a second electrical terminal on said base portion, a plastic film on said support member, said plastic film including an electrolytic and an hygroscopic material, whereby a mass ionization throughout said plastic film occurs when said film is exposed to moisture, and first and second electrical conductors connected to said first and second terminals, respectively, and embedded in said film in a spiral convolution about said support member.

2. A humidity sensitive element comprising a support member having a base portion, at least a pair of electrical terminals connected to said base portion, a polymerized layer on said support member, said layer being electrolytic and hygroscopic in nature, at least a pair of electrical conductors connected to said terminals and embedded in said layer in a spiral convolution about said support member, whereby a mass ionization throughout said layer occurs when said layer is exposed to moisture, said mass ionization facilitating a current carrying capacity for said element in the milliamps range.

3. A humidity sensitive element having a base portion and comprising, electrical terminal means connected to said base portion, a polymerized film on said base portion, said film being electrolytic and hygroscopic in nature, electrical conductor means connected to said terminal means and embedded in said film, whereby a mass ionization throughout said film occurs when said film is exposed to moisture, said mass ionization facilitating a current carrying capacity for said element in the milliamp range.

4. A humidity sensitive element comprising, electrical terminal means, a polymerized self-supporting film, said film being electrolytic and hygroscopic in nature, electrical conductor means connected to said terminal means and embedded in said film, whereby a mass ionization through said film occurs when said film is exposed to moisture, said mass ionization facilitating a current carrying capacity for said elementin the milliamps range.

5. A humidity sensitive element comprising a support member having a base portion, at least a pair of electrical terminals connected to said base portion, a polymerized layer on said support member, said layer being electrolytic and hygroscopic in nature, at least a pair of electrical conductors connected to said terminals and embedded in said layer in a spiral convolution about said support member, whereby a mass ionization throughout said layer occurs when said layer is exposed to moisture, said mass ionization facilitating a current carrying capacity for said element in the milliamps range.

6. A humidity sensitive element comprising, electrical terminal means, a polymerized film being electrolytic and hygroscopic in nature, an electrical conductor means in said film connected to said electrical terminal means, whereby a mass ionization throughout said film occurs when said film is exposed to moisture, said mass ionization facilitating a current carrying capacity for said element in the range of approximately ten milliamps.

7. A humidity sensitive element comprising, electrical terminal means, a polymerized film being electrolytic and hygroscopic in nature, an electrical conductor means in said film connected to said electrical terminal means,

E 1 whereby a mass'ionization throughout said film occurs when said film is exposed to moisture, said mass ionization facilitating a current carrying capacity for said element in the milliamps range.

8. A humidity sensitive element comprising an insoluble polymerized film capable of attracting moisture and having ionizable material homogeneously dispersed throughout the film in fixed relation to the film to place the ionizable material in continuous phase, said film throughout being electrolytic in nature in the presence of moisture, and electrical conductor means embedded in said film, whereby a mass ionization throughout said film occurs when said film is exposed to said moisture.

References Cited in the file of this patent UNITED STATES PATENTS McCorkhill May 28, Gillingham May 30, Pope Dec. 27, Horecky Jan. 21, Reiley July 21, Weston et a1 Mar. 22, Richards June 14, Beerman Jan. 3, Brogan et a1. Mar. 28, Crabtree I an. 15, 

1. A HUMIDITY SENSITIVE ELEMENT COMPRISING A CYLINDRICAL SUPPORT MEMBER HAVING AN INTEGRAL ANNULAR FLANGED BASE PORTION, A FIRST ELECTRICAL TERMINAL ON SAID BASE PORTION, A SECOND ELECTRICAL TERMINAL ON SAID BASE PORTION, A PLASTIC FILM ON SAID SUPPORT MEMBER, SAID PLASTIC FILM INCLUDING AN ELECTROLYTIC AND AN HYGROSCOPIC MATERIAL, WHEREBY A MASS IONIZATION THROUGHOUT SAID PLASTIC FILM OCCURS WHEN SAID FILM IS EXPOSED TO MOISTURE, AND FIRST AND SECOND ELECTRICAL CONDUCTORS CONNECTED TO SAID FIRST AND SECOND TERMINALS, RESPECTIVELY, AND EMBEDDED IN SAID FILM IN A SPIRAL CONVOLUTION ABOUT SAID SUPPORT MEMBER. 